Although a variety of systems for dry denitrification have been hitherto proposed, most of them utilize a moving-bed reactor in which particulate catalysts are used. This is because a packed-bed reactor utilizing particulate catalysts has a disadvantage that the catalyst bed is blocked by foreign materials such as dust which are contained in the gas to be treated. As one way for overcoming such a disadvantage, it was proposed to use a "dust-free" catalyst. However, dust-free catalysts have not been put into a practical use since most of the dust-free catalysts do not yet exhibit a sufficient resistance to dusts and further, dust-free catalysts generally have a low catalytic surface area per volume and hence need to be loaded in a larger amount as compared with particulate catalysts.
However, along with development of more active dust-free catalysts, a denitrification device utilizing dust-free catalyst has been recently recognized because of its structural simplicity and easiness of operation. There are a variety of shapes of dust-free catalysts, some typical examples being pipe-shaped catalyst, honeycomb-shaped catalyst, plate-shaped catalyst and so on. Among these, pipe-shaped catalyst is the most advantageous, as compared with the other dust-free catalysts, particularly with honeycomb-shaped catalyst, because of its lower cost since it is easy to be shaped, has low loss of catalyst material in shaping and further, can be manufactured with the use of a relatively small-scale device. With the recent enlargement of dry denitrification systems, the cost of the catalyst and the cost of the reactor have come to contribute more to the overall cost of the system. Therefore, a pipe-shaped catalyst, because of its low cost, is suitable for use particularly in a large-scale device for dry denitrification.
Although pipe-shaped catalyst is highly rated in the economical sense as stated above, it has disadvantages due to the arrangement thereof in the reactor. As seen from FIGS. 1(A) and (B) showing partial cross sectional views of reactors, pipe-shaped catalysts have been hitherto arranged in reactors in the square closest packing mode (A) or triangular closest packing mode (B). The gas to be treated is to pass through flow passages as defined by the inner surfaces of the pipe-shaped catalysts (the passages designated as 1 or 1') and also through flow passages as defined by the outer surfaces of the pipe-shaped catalysts (the passages designated as 2 or 2'). However, the flow passages as defined by the inner surfaces and the flow passages as defined by the outer flow passages are greatly different from each other in their cross sectional shapes as seen from FIGS. 1 (A) and (B), and hence, the states of the gas flowing through such two types of passages are greatly different. This will be clear from the following fact.
FIG. 2, FIG. 3 and FIG. 4 show the results of fluid dynamic experiments carried out by the present inventors, for the case where pipe-shaped catalysts, each having an inside diameter of 21 mm and an outside diameter of 32 mm, are arranged in the square closest packing mode shown in FIG. 1(A).
The graph of FIG. 2 shows the relationship between the Reynolds number (Re) and the friction factor (f) of the gas flowing in the flow passages as defined by the inner surfaces of the pipe-shaped catalyst. The graph of FIG. 2 clearly shows the characteristic of turbulent flow in a rough-wall pipe, as is well known in fluid dynamics.
The graph of FIG. 3 shows the relationship between the Reynolds number (Re) and the friction factor (f) of the gas flowing in the flow passages as defined by the outer surfaces of the pipe-shaped catalysts. From FIG. 3, it is understood that the gas flow is laminar in the outer surface flow passages.
FIG. 2 and FIG. 3 show the results of experiments made with the same pressure drop across the outer surface flow passages and across the inner surface flow passages. In a practical reactor for dry denitrification, the pressure drop across the outer surface flow passages and that across the inner surface flow passages are equal. Thus, from the experimental data as shown in FIG. 2 and FIG. 3, it is understood that, in a reactor in which the catalysts are arranged as shown in FIG. 1 (A), the gas flows in turbulent flow through the inner surface flow passages and in laminar flow through the outer surface flow passages.
FIG. 4 shows the experimental results (those as shown in FIG. 2 and FIG. 3) in terms of the relationship between linear velocity of the gas flowing through the passages and pressure loss per meter of the length of the passages. From FIG. 4, it is understood that for yielding a given pressure loss, the gas velocity for the inner-surface flow passages (graph 5) and that for the outer-surface flow passage (graph 6) are much different from each other. Further, since the cross-sectional area of an inner-surface flow passage is approximately 346 mm.sup.2 and the cross sectional-area of an outer-surface flow passage is approximately 218 mm.sup.2, the difference in terms of the gas flow rate (linear velocity multiplied by cross sectional area) is greater.
As shown in FIG. 2 through FIG. 4, although the total catalytic surface area of the outer-surface passages are larger than that of the inner-surface flow passages, the gas flows through the outer-surface flow passages at a smaller flow rate and further in a laminar flow. It can be therefore said that the outer-surface flow passages are not sufficiently utilized as catalyst and, from the stand point of efficient utilization of the catalyst, there is too much loss in such conventional modes of catalyst arrangement as the square closest packing arrangement mode.
Furthermore, in a case where the gas velocity is extremely low locally in the flow passages, there is a risk that the dust contained in the gas will accumulate so as to lead the blockage of the flow passages in addition to the lowering of catalytic activity. As a matter of fact, it was confirmed through the experiments of the present inventors that such dust accumulated in the circumferential portions in the outer-surface flow passages.
As a way for overcoming the above-mentioned disadvantages, i.e., for equalizing the flow states of the gas in the two types of flow passages, it may be proposed to make the shapes of the two passages to be hydro-dynamically similar and more particularly, as a concrete and simple answer, to arrange pipe-shaped catalysts so that the hydraulic diameters of the two passages are equal. This solution is based on a hydrodynamic assumption that the friction factor for all of the flow passages are equal. However, as seen from FIG. 2 and FIG. 3 based on the experiments of the present inventors and showing that the friction factor for the outer-surface flow passages is different from friction factor for laminar flow in a cylindrical tube, when the geometrical shapes of flow passages are different, the friction factors for the passages are different. In addition, as will be explained later, the friction factor is closely related to the catalytic reaction rate constant. Therefore, from the standpoint of catalytic reaction engineering, such a determination of the distance between the catalysts, in disregard of the difference in friction factors, will not achieve any practical effects.